C. Geisik

Microwave proton source development for a high‐current linac injector (invited)a)

Powerful CW proton linear accelerators (100-mA at 0.5--1.0 GeV) are being proposed for spallation neutron-source applications. A 75-keV, 110-mA dc proton injector using a microwave ion source is being tested for these applications. It has achieved 80-keV, 110-mA hydrogen-ion-beam operation. Video and dc beam-current toroid diagnostics are operational, and an EPICS control system is also operational on the 75-keV injector. A technical base development program has also been carried out on a 50-keV injector obtained from Chalk River Laboratories, and it includes low-energy beam transport studies, ion source lifetime tests, and proton-fraction enhancement studies. Technical base results and the present status of the 75-keV injector will be presented.

H− temperature measurements by a slit diagnostic technique

H/sup -/ ion beams are extracted at 5-25 kV from a long, narrow slit on a Penning surface-plasma source (the 8X source). The extraction geometry produces negligible transverse electric field (focusing effects) along the slit length. Therefore, the ion angular spread reflects the distribution of ion energies at the plasma surface. The angular distributions are measured with an electric-sweep emittance scanner whose slits are oriented normal to the long dimension of the emission slit. The nearly Maxwellian angular distributions measured over the central portions of the ribbon beam give kT(H/sup -/) of 0.1 eV to 0.2 eV for a 2-A DC discharge and 0.8 eV to 1.0 eV for 350-A to 500-A pulsed discharges. This diagnostic technique has sufficient position resolution to allow measurement of the kT(H/sup -/) spatial distributions. It also allows study of the kT(H/sup -/) dependencies on ion source parameters (e.g. increasing the H/sub 2/ gas flow lowers kT(H/sup -/)).<<ETX>>

Heavy beam loading measurements of an rf accelerating cavity under amplitude and phase control

A technique for measuring both resistive and reactive particle‐beam‐loading effects on an rf accelerating cavity is described. A high‐power, six‐port reflectometer is used to measure the complex cavity impedance, both with and without beam, as a function of cavity resonant frequency. Resistive and reactive beam loading appears as a decrease in the cavity quality factor Q and as a change in the cavity resonant frequency, respectively. An equivalent circuit model for the drive system, cavity, and beam is used to quantify these effects as a function of cavity voltage and phase, beam current, and cavity detuning. Measurements on a heavily beam‐loaded two‐cell drift‐tube linac are compared with predictions from the equivalent circuit model and good agreement is found. The measurements are performed while maintaining constant cavity‐field amplitude and phase. Thus, this technique is a nonperturbative measurement.

H- temperature dependences in a Penning surface-plasma source

Simple analysis of the nearly-Maxwellian angular distribution of the ribbon H{sup {minus}} ions beams extracted from a long, narrow slit on the 8X source yields the H{sup {minus}} temperature, kT{sub H{minus}}. The derived kT{sub H{minus}} are 0.1--0.3 eV for a 2-A dc discharge and 0.7-1.3 eV for a 400-A pulsed discharge. Because this diagnostic method relies on simple electronic techniques, it allows rapid study of the dependencies of kT{sub H{minus}} on the source parameters, such as gas flow and discharge current. These variations of kT{sub H{minus}} in the 8X source are qualitatively similar to those observed for the H-atom temperature, kT{sub H}o, in the 4X source, another Penning surface-plasma source. 10 refs., 4 figs.

CW 8X ion source development

Using the 4X source performance and the Penning SPS scaling laws, we predicted the performance of the 8X source. A pulsed 8X source was then built and tested. After verifying the pulsed 8X source operation, especially the H− beam current, emittance, and power efficiency, we designed and built the CW 8X source. We plan to operate the source arc at dc power levels up to 30 kW. This will be accomplished by actively cooling the electrode surfaces with pressurized, hot water. The CW 8X source is presently undergoing shake‐down tests on a test stand at Los Alamos. Evaluation of the high‐current, hydrogen‐cesium dc arc will begin when these tests are completed.

A direct‐current Penning surface‐plasma source

After developing a pulsed 8X source for H− beams, we are now testing a cooled, dc version. The design dc power density on the cathode surface is 900 W/cm2, much higher than achieved in any previously reported Penning surface‐plasma source. The source is designed to accommodate dc arc power levels up to 30 kW by cooling the electrode surfaces with pressurized, hot water. After striking the arc using a 600‐V pulser, a 350‐V dc power supply is switched in to sustain the 100‐V discharge. Now our tests are concentrating on arc pulse lengths ≤1 s. Ultimately, the discharge will be operated dc. The source is described and the initial arc test results are presented.

Initial operation of the CW 8X H/sup -/ ion source discharge

A pulsed 8X source was built and the H/sup -/ beam current, emittance, and power efficiency were measured. These results were promising, so a cooled, dc version designed for operation at are power levels up to 30 kW was built. Testing of the CW 8X source discharge is underway. The design dc power loading on the cathode surface is 900 W/cm/sup 2/, considerably higher than achieved in any previous Penning surface-plasma source (SPS). Thus, the electrode surfaces are cooled with pressurized, hot water. We describe the source and present the initial operating experience and are test results.<<ETX>>

DEVELOPMENT OF A TOROIDAL-FILTER H- ION SOURCE

High brightness H− ion beams will be required for the next generation of accelerator‐driven, neutron spallation sources. Volume H− ion sources have the potential for providing these beams at a high duty factor with moderate beam currents and with the long‐term operational stability and reliability needed. We report on the results of a continuing study of the Los Alamos National Laboratory version of a toroidal‐filter, volume H− ion source which was designed to provide such beams. The H− beam current, emittance, and electron loading have been measured with the source equipped with a 3‐ and a 10‐mm‐diam emission aperture and with several configurations of the plasma filter. The results are compared to beam simulations.

Development of a volume H/sup -/ ion source for LAMPF

We have continued the development of a toroidal-filter volume H/sup -/ ion source with the goal of providing a better ion source for the proton storage ring at the Los Alamos Neutron Scattering Center (LANSCE). A prototype source has produced H/sup -/ beam currents of 6.3 mA from a 3-mm-diameter emission aperture (89 mA/cm/sup 2/) and 18 mA from a 10-mm-diameter emission aperture (23 mA/cm/sup 2/). These results were achieved when cesium was added to tile discharge. The cesium increased the H/sup -/ current by a factor of 2-3 while the extracted electron current was strongly suppressed. The magnetic filter can be configured so that the electron to H/sup -/ ratio is 2:1 and is weakly dependent on operating parameters, with only a moderate loss of H/sup -/ current. Tests indicate there is essentially no explicit dependence of extracted beam current on duty factor up to 10% duty-factor. Emittance data were taken for various operating conditions in a mass-analyzed beam line and a comparison is made to the surface conversion H/sup -/ ion source now in operation for the storage ring.

H - temperature measurements by a slit diagnostic technique

H/sup -/ ion beams are extracted at 5-25 kV from a long, narrow slit on a Penning surface-plasma source (the 8X source). The extraction geometry produces negligible transverse electric field (focusing effects) along the slit length. Therefore, the ion angular spread reflects the distribution of ion energies at the plasma surface. The angular distributions are measured with an electric-sweep emittance scanner whose slits are oriented normal to the long dimension of the emission slit. The nearly Maxwellian angular distributions measured over the central portions of the ribbon beam give kT(H/sup -/) of 0.1 eV to 0.2 eV for a 2-A DC discharge and 0.8 eV to 1.0 eV for 350-A to 500-A pulsed discharges. This diagnostic technique has sufficient position resolution to allow measurement of the kT(H/sup -/) spatial distributions. It also allows study of the kT(H/sup -/) dependencies on ion source parameters (e.g. increasing the H/sub 2/ gas flow lowers kT(H/sup -/)). >